Electrohydraulic proportional pressure control for open circuit pump

ABSTRACT

A pump control assembly for controlling a variable displacement hydraulic pump includes a spool mounted within a valve block. The spool is configured to move between a first and a second position within the valve block so as to selectively control the displacement of the attached pump. The pump control assembly further includes first and second chambers that each apply a force to opposite ends of the spool. The first chamber is positioned at a first end of the spool in fluid communication with a pump output port. The second chamber is positioned at a second end of the spool and in fluid communication with a hydraulic tank port and a proportional pressure reducing valve. The second chamber also includes a piston and first and second springs positioned on either side of the piston. The proportional pressure reducing valve provides a regulated pressure to a first side of the piston along with the first spring, and the hydraulic tank port provides a tank pressure on the opposite side of the piston along with the second spring. The pump control assembly also includes a stop structure having a positive stop that limits movement of the piston in a direction toward the first chamber.

CROSS-REFERENCE TO RELATED APPLICATION

This application is being filed on Aug. 9, 2016 as a PCT InternationalPatent Application and claims the benefit of Indian Patent ApplicationNo. 2449/DEL/2015, filed on Aug. 10, 2015, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND

Controlling the output flow of a variable displacement pump is importantto maintain a stable hydraulic system. Doing so with accuracy can helpprotect the system from unintended damage and can aid in improving theoverall efficiency of the hydraulic system.

Variable displacement pumps, specifically axial piston pumps, generallyinclude a drive shaft, a cylinder barrel that is rotatable by the driveshaft, multiple piston bores positioned about the cylinder barrel, andmultiple pistons positioned within the piston bores and attached to atiltable swash plate. To control the displacement of the axial pistonpump, the angle of the swash plate must be altered. Traditionallychanging the angle is accomplished by a swash plate piston cylinder orsolenoid. When the swash plate is tilted relative to the longitudinalaxis of the drive shaft, the pistons reciprocate within the piston boresto produce a pumping action. Therefore, the larger the swash plateangle, the larger the displacement of the pump.

When controlling the swash plate piston cylinder or solenoid, thepressure from the hydraulic tank and the pressure from the hydrauliccircuit are typically considered. For example, if a hydraulicspring-loaded piston cylinder is used to control the angle of the swashplate, tank pressure can act on one side of the piston and hydrauliccircuit pressure can act on the other side of the piston. Depending onthe difference between the two pressures and the spring constant, thepiston will move within the cylinder accordingly. Because the piston isalso attached to the swash plate, as the pressure difference changes andmoves the piston, the swash plate angle also changes, thereby changingthe displacement of the pump.

In other examples, when the swash plate is controlled by the action of asolenoid, the change in displacement of the pump is commonlyproportional to the current supplied to the solenoid by a controller.

Customized real time control of the displacement of the pump is oftendesired. Therefore, the piston cylinders or solenoids often areconfigured to allow for the on-demand altering of the pump displacement.Additionally, hydraulic pressure within the hydraulic circuit can changeabruptly during operation. Such changes can be caused by a failure,excess load, etc. Additionally, electronics controlling the displacementof the pump (i.e., by solenoid) can also fail, causing a drasticincrease in pressure. Therefore, a separate pressure compensator deviceis often included as part of the system to safeguard the system in suchscenarios.

Improvements in variable displacement pump control are desired.

SUMMARY

The present disclosure relates to pressure control. More particularly,the disclosure relates to proportionally controlling the outflow of apiston pump in an open hydraulic circuit.

In accordance with an aspect of the disclosure, a pump control assemblyfor controlling a variable displacement hydraulic pump is disclosed. Thepump control assembly for controlling a variable displacement hydraulicpump includes a spool mounted within a valve block. The spool beingconfigured to move between a first and a second position within thevalve block so as to selectively control the displacement of theattached pump. The pump control assembly further includes first andsecond chambers that each apply a force to opposite ends of the spool.The first chamber is positioned at a first end of the spool in fluidcommunication with a pump output port. The second chamber is positionedat a second end of the spool and in fluid communication with a hydraulictank port and a proportional pressure reducing valve. The second chamberalso includes a piston and first and second springs positioned on eitherside of the piston. The proportional pressure reducing valve provides aregulated pressure to a first side of the piston along with the firstspring, and the hydraulic tank port provides a tank pressure on theopposite side of the piston along with the second spring. The pumpcontrol assembly also includes a stop structure having a positive stopthat limits movement of the piston in a direction toward the firstchamber.

In accordance with another aspect of the disclosure, a pump controlassembly for controlling a variable displacement hydraulic pump isdisclosed. The pump control assembly for controlling a variabledisplacement hydraulic pump includes a valve block that defines a spoolbore that has a central bore axis. The valve block also defines a pumpoutput port, a pump displacement control port, and a tank port. The pumpcontrol assembly also includes a spool mounted within the spool bore.The spool has a first end and an opposite second end and is movablewithin the spool bore along the central bore axis between a firstposition where the tank port is in fluid communication with the pumpdisplacement control port and a second position in which the pump outputport is in fluid communication with the pump displacement control port.The spool moves in a first direction along the central bore axis whenmoving from the first position toward the second position. The spoolmoves in a second direction along the central bore axis when moving fromthe second position toward the first position. The first and seconddirections are opposite from one another.

The pump control assembly also includes a first chamber positioned atthe first end of the spool. The first chamber is in fluid communicationwith the pump output port so as to be configured to receive a pumpoutput pressure from the variable displacement pump when the pumpcontrol assembly is installed on the variable displacement pump. Whenthe pump output pressure is applied to the first chamber, the pumpoutput pressure applies a pump output pressure force to the spool in thefirst direction. The pump control assembly also includes a secondchamber positioned at the second end of the spool. A piston ispositioned within the second chamber so as to divide the second chamberinto a first section and a second section. The first section ispositioned between the piston and the second end of the spool, and thepiston is movable within the second chamber along the central bore axis.

Further, the pump control assembly includes a first spring positionedwithin the first section of the second chamber for transferring a pistonforce in the second direction from the piston to the spool. A secondspring is positioned within the second section of the second chamber forapplying a pre-load force to the piston for biasing the piston in thesecond direction. The pump control assembly also includes a proportionalpressure reducing valve mounted within the valve block. The proportionalpressure reducing valve is operable in a first state where the tank portis in fluid communication with the second section of the second chamberand a second state where the pump output port is in fluid communicationwith the second section of the second chamber. The proportional pressurereducing valve is configured to convert the pump output pressure into areduced pressure that is provided at the second section of the secondchamber. The reduced pressure at the second section of the secondchamber acts on the piston to apply a reduced pressure force to thepiston in the second direction, and the magnitude of the reducedpressure output from the proportional pressure reducing valve isdirectly proportional to a current provided to a solenoid of theproportional pressure reducing valve.

Additionally, a stop structure is included in the pump control assembly.The stop structure has a positive stop that stops movement of the pistonin the second direction along the central bore axis at a stop positionthat defines a maximum threshold for the piston force transferred by thefirst spring in the second direction from the piston to the spool. Thestop position is adjustable along the central bore axis to adjust themaximum threshold of the piston force.

A variety of additional aspects will be set forth in the descriptionthat follows. The aspects can relate to individual features and tocombinations of features. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory only and are not restrictive of the broad inventiveconcepts upon which the embodiments disclosed herein are based.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent disclosure and therefore do not limit the scope of the presentdisclosure. The drawings are not to scale and are intended for use inconjunction with the explanations in the following detailed description.Embodiments of the present disclosure will hereinafter be described inconjunction with the appended drawings, wherein like numerals denotelike elements.

FIG. 1 illustrates a schematic view of a an example hydraulic systemhaving features that are examples of inventive aspects in accordancewith the principles of the present disclosure;

FIG. 2 illustrates a perspective view of an electro-hydraulicproportional pressure control valve having features that are examples ofinventive aspects in accordance with the principles of the presentdisclosure;

FIG. 3 illustrates a bottom view of the electro-hydraulic proportionalpressure control valve of FIG. 2;

FIG. 4 illustrates an exploded view of the electro-hydraulicproportional pressure control valve of FIG. 2;

FIG. 5 illustrates a side view of the electro-hydraulic proportionalpressure control valve of FIG. 2;

FIG. 6 illustrates a cross-sectional view along line 6-6 in FIG. 5 ofthe electro-hydraulic proportional pressure control valve of FIG. 2; and

FIG. 7 illustrates a cross-sectional view along line 7-7 in FIG. 5 ofthe electro-hydraulic proportional pressure control valve of FIG. 2.

DETAILED DESCRIPTION

Various embodiments will be described in detail with reference to thedrawings, wherein like reference numerals represent like parts andassemblies throughout the several views. Reference to variousembodiments does not limit the scope of the claims attached hereto.Additionally, any examples set forth in this specification are notintended to be limiting and merely set forth some of the many possibleembodiments for the appended claims.

In general, an electro-hydraulic proportional pressure control valve(EHPPCV) for an axial piston pump is disclosed. In particular, theEHPPCV uses a hydraulically controlled spool valve to control the flowof fluid to the swash plate piston, thereby controlling the displacementof the pump. The spool valve's movement is determined by the relativedifference in forces exerted by a spring-loaded piston at one end of thespool and the pump outlet pressure exerting a force at an opposite endof the spool. Forces within the spring-loaded piston can be varied by apair of springs and a regulated pressure force supplied by a highpressure proportional pressure reducing valve that is controlled by anelectric solenoid. Additionally, to safe guard the system, a mechanicalstop is provided within the EHPPCV to prevent the spring loaded pistonfrom exerting too high of a pressure on the spool valve, thereby leadingto potentially over stroking the pump and possibly damaging thehydraulic circuit. Such a stop prevents the need to include a separatepressure compensator to safe guard the system. Also, the EHPPCV isconfigured to provide a step-less pressure control of the axial pistonpump, thereby increasing the overall system stability.

FIG. 1 shows a hydraulic schematic of an example hydraulic system 100.The hydraulic system 100 includes a pump 102, a hydraulic fluid tank104, a swash plate piston 106, and an EHPPCV arrangement 108.

The pump 102 is an axial piston pump. The pump 102 receives power by wayof a drive shaft 110. The pump 102 is fluidly connected to the tank 104and configured to pump fluid from the tank 104 to a hydraulic circuit112.

The displacement of the pump is altered by the swash plate piston 106.The swash plate piston 106 is configured to change the angle of a swashplate 107 within the pump 102, thereby changing the displacement of thepump 102. As the pump displacement changes, the outlet pressure of thepump changes. A change in outlet pressure changes the fluid pressurewithin the hydraulic circuit 112. The position of the swash plate piston106 is controlled by the EHPPCV arrangement 108.

The EHPPCV arrangement 108 includes a solenoid powered valve 114 and aspring-loaded piston cylinder 116. Both the solenoid powered valve 114and the spring loaded piston cylinder 116 are configured to receivehydraulic fluid flow from the outlet of the pump 102 that is indicativeof the flow and pressure of the hydraulic circuit 112. A forcecorresponding to a regulated pressure directed from the solenoid poweredvalve 114 and a force corresponding to the spring-loaded piston cylinder116 are configured act on a single side of a spool valve 115.

The solenoid powered valve 114 is configured to provide a flow of fluidto the spring-loaded cylinder 116. The solenoid powered valve 114 isconfigured to receive an electrical current from a controller (notshown) and adjust the fluid flow leaving the solenoid powered valve 114according to the magnitude of the electrical current. In the depictedembodiment, the solenoid powered valve 114 is configured to receivefluid from the hydraulic circuit 112 that has a pressure that is equalto the pump output pressure. The solenoid powered valve 114 thenconverts the fluid at pump output pressure to a regulated pressure (line122). In some embodiments, the regulated pressure is a reduced pressurerelative to the pressure of the hydraulic circuit 112. In the depictedembodiment, the solenoid powered valve 114 is a proportional pressurereducing valve; therefore, the regulated pressure output (line 122) fromthe solenoid powered valve 114 is directly proportional to a currentprovided to the solenoid powered valve 114 by a controller.Additionally, in the depicted embodiment, the displacement of the pump102 is also directly proportional to the current supplied to thesolenoid powered valve 114.

The spring-loaded piston cylinder 116 is configured to exert a force onone side of the spool valve 115. The spring-loaded piston cylinder 116'sbehavior is influenced by a first spring 118, a second spring 120 and bytwo separate fluid pressures. The two separate fluid pressures includethe regulated fluid pressure (line 122) received from the solenoidpowered valve 114 and a tank fluid pressure (line 124) which isrepresentative of the pressure within the tank 104. The operation of theEHPPCV arrangement 108 will be explained in more detail with respect toFIGS. 5-6.

The spool valve 115 is shown to be in fluid communication with thesolenoid powered valve 114, the spring-loaded piston cylinder 116, andthe hydraulic circuit 112. The hydraulic circuit 112 provides a pressureindicative of the outlet pump pressure at one side of the spool valve115, while the solenoid powered valve 114 and the spring-loaded pistoncylinder 116 provide a pressure at the opposite side of the spool valve115. Accordingly, the spool valve 115 is configured to deliver a fluidflow (line 126) to the swash plate piston 106, thereby manipulating theswash plate piston 106 to achieve a desired displacement of the pump102.

FIG. 2 shows a perspective view of the EHPPCV arrangement 108. TheEHPPCV arrangement 108 can be mounted to the pump 102 or in a separatelocation. In some embodiments, the EHPPCV arrangement 108 is mountedwithin a housing (not shown) of the pump 102.

As shown, the EHPPCV arrangement 108 includes a valve block 128 that isconfigured to house the spring-loaded piston cylinder 116, the solenoidpowered valve 114, and the spool valve 115. The valve block 128 has afirst side 129 and a second side 131. Additionally, as shown in a bottomview of the EHPPCV arrangement 108 in FIG. 3, the valve block 128includes a plurality of ports that include a pump output port 133, apump displacement control port 135, and a tank fluid port 137. The pumpoutput port 133 is configured to be fluidly connected to the hydrauliccircuit 112, the pump displacement control port 135 is configured to befluidly connected to the swash plate piston fluid line 126, and the tankfluid port 137 is configured to be fluidly connected to the tank fluidline 124.

FIG. 4 shows an exploded view of the EHPPCV arrangement 108. The EHPPCVarrangement 108 is shown to include the spring-loaded piston cylinder116, solenoid powered valve 114, the spool valve 115, a stop structure132, and a plurality of plugs.

The spring-loaded piston cylinder 116 is shown to include a plunger 134,the first spring 118, a piston 136, the second spring 120, and a pistonchamber plug 139. The plunger 134, first spring 118, piston 136, andsecond spring 120 are all housed within a piston chamber 138 defined bythe valve block 128 and sealed within the piston chamber 138 by thepiston chamber plug 139. The piston chamber plug 139 seals within thepiston chamber 138 by way of a seal 142 and is secured to the valveblock 128 by a lock nut 144.

The solenoid powered valve 114 includes a solenoid 146, a valve shaft148, and a connection plug 150 for attaching an electrical connection tothe controller. The solenoid powered valve 114 is configured to beconnected to the valve block 128 and seated in a solenoid powered valvechamber 152 defined by the valve block 128.

The spool valve 115 includes a spool 130 mounted within the valve block128 (shown in detail in FIG. 6). The spool 130 is configured to controlthe flow of fluid to the swash plate piston 106 by way of the pumpdisplacement control port 135.

The stop structure 132 is also mounted within the valve block 128. Thestop structure 132 is configured to limit the movement of the piston 136within the piston chamber 138. The stop structure 132 includes a firstshaft 154, a second shaft 156, and a locking nut 158.

FIG. 5 shows the EHPPCV arrangement 108 from the first side.Specifically, the stop structure 132 is shown installed within the valveblock 128. As shown, the second shaft 156 and the locking nut 158 areshown positioned outside of the valve block 128. A spool plug 160 isalso shown and is configured to seal a spool bore (shown in FIG. 6) inwhich the spool 130 is housed within the valve block 128.

FIG. 6 shows a cross-sectional view of the EHPPCV arrangement 108 alongline 6-6 in FIG. 5. Due to the fact that a single cross section cannotbe used to capture all fluid passageways and cavities within the valveblock 128, dotted lines are used to show the fluid paths within theEHPPCV arrangement 108.

The spool 130 of the spool valve 115 is shown movably mounted within aspool bore 162 within the valve block 128. The spool 130 includes acentrally positioned land structure 131 that acts to control the flow offluid through the pump displacement control port 135. The spool bore 162defines a spool axis 163, and the spool 130 is movable along the axisbetween a first position and a second position. When in the firstposition, the tank fluid port 137 is in fluid communication with thepump displacement control port 135, and when in second position, thepump output port 133 is in fluid communication with the pumpdisplacement control port 135. In FIG. 6, the spool 130 is shown in thefirst position. The spool 130 can move between the first and secondpositions. In the first position, due to the lower relative pressure inthe tank 104 with respect to the hydraulic circuit 112, a lower pressurefluid is supplied to the swash plate piston 106. In the second position,a higher fluid pressure from the hydraulic circuit 112 is supplied tothe swash plate piston 106 causing the swash plate piston 106 to move,thereby decreasing the angle of the swash plate within the pump 102 andlowering the displacement (i.e., de-stroke) of the pump 102.

To facilitate the movement of the spool 130 between the first and secondpositions, a first chamber 168 and the piston chamber 138 are positionedat either end 164, 166 of the spool 130 of the spool valve 115. Eachchamber 168, 138 is configured to exert a force on the spool 130 inorder to influence the spool's movement.

The first chamber 168 is positioned at the first end 164 of the spool130 and is in fluid communication with the pump output port 133, andthereby the hydraulic circuit 112. The first chamber 168 receives apressurized fluid that is representative of the pump outlet pressurewhen the EHPPCV arrangement 108 is installed on the pump 102. Thepressure within the first chamber 168 exerts a force on the spool 130 ina direction D1 toward the second side 131 of the valve block 128.

The piston chamber 138 is positioned at the second end 166 of the spool130. The piston chamber 138 is configured to exert a force on the spool130 in a direction D2 toward the first side 129 of the valve block 128.The piston chamber includes a first section 170 and second section 172,divided by the piston 136. In the depicted embodiment, a force isexerted on the plunger 134 by the first spring 118. A force is exertedon the first spring 118 by the piston 136, and a force is exerted on thepiston 136 by the second spring 120. Additionally, the first and secondsections 170, 172 of the piston chamber 138 are configured to receive afluid pressure to further change the overall force applied to the spool130 by the piston chamber 138.

The piston 136 is movable within the piston chamber 138. As forceschange within the first section 170 and second section 172, the pistonmoves within the piston chamber 138, thereby changing the overall forceexerted on the plunger 134 on the second end 166 of the spool 130 bycompressing or decompressing the first spring 118. The piston 136 canalso seal the first section 170 from the second section 172, so as toallow each section to maintain different pressures and forces.

The first section 170 of the piston chamber 138 is positioned betweenthe second end 166 of the spool 130 and the piston 136 within the pistonchamber 138. The first section 170 is shown to include the first spring118 and the plunger 134. Also, the first section 170 is shown to be influid communication with the tank fluid port 137, thereby receiving atank pressure. The tank pressure and first spring 118 exert a resistanceforce on the piston 136 in a direction toward the second side 131 of thevalve block 128. In some embodiments, the tank pressure will be close tozero and the first spring 118 will exert the only force on the plunger134 and piston 136

The second section 172 of the piston chamber 138 includes the secondspring 120 positioned between the piston 136 and the piston chamber plug139. The second spring 120 is configured to apply an adjustable preloadforce to the piston 136. The force exerted by the second spring 120 canbe altered by the position of the piston chamber plug 139 within thepiston chamber 138. Such positioning is adjustable by the user and thepiston chamber plug 139 can be secured with respect the piston chamber138 by way of the lock nut 144.

The second section 172 is also configured to receive a regulatedpressure from the solenoid powered valve 114. In some embodiments, thesolenoid powered valve 114 is configured to deliver between zeropressure and a pressure that is greater than tank pressure but less orequal to than the pump outlet pressure. When there is a balance of fluidpressures between the first section 170 and the second section 172, theforce that is exerted on the plunger 134 is equal to the force exertedby the first spring 118. Such a force can be altered by altering theforce of the second spring, which either compresses or decompressing thefirst spring 118 by allowing the piston 136 to move within the pistonchamber 138.

The solenoid powered valve 114 is configured to be in fluidcommunication with both the pump output port 133 and the tank fluid port137. The current delivered to the solenoid 146 of the solenoid poweredvalve 114 can be varied so as to change the positioning of the valveshaft 148. Changing the positioning of the valve shaft 148 changes theregulated fluid pressure in the regulated fluid pressure cavity 174. Theregulated fluid pressure is then delivered from the regulated fluidpressure cavity 174 to the second section 172 of the piston chamber 138.The magnitude of the regulated fluid pressure from the solenoid poweredvalve 114 is directly proportional to a current provided to the solenoid146.

FIG. 7 shows a cross-sectional view of the EHPPCV arrangement 108 alongline 7-7 in FIG. 5. Specifically, FIG. 7 shows the stop structure 132that includes the first shaft 154, second shaft 156, and locking nut158. The stop structure 132 is mounted within a stop structure bore 176.The stop structure bore 176 is parallel to the spool bore 162. The stopstructure 132 includes a positive stop 178 that prevents the movement ofthe piston 136 past the positive stop 178 in a direction toward thefirst side 129 of the valve block 128. By limiting the movement of thepiston 136, the maximum threshold for the force that the piston 136transfers to the first spring 118 in the direction toward the first side129 is limited. Additionally the location of the positive stop 178 canbe altered. When installed within the valve block 128, the locking nut158 and second shaft 156 are accessible from outside of the valve block128 so that the location of the positive stop 178, and therefore theoverall stop structure 132, can be adjusted. In some embodiments, thestop structure 132 is threaded into the valve block 128 and can berotated for adjustment.

The stop structure 132 helps to prevent the over pressurizing of theswash plate piston 106, thereby preventing the pump 102 from overpressurizing the hydraulic circuit 112. The stop structure 132 is asafety device to prevent the hydraulic circuit 112 from being damagedinadvertently. Because the force exerted on the second end 166 of thespool 130 in the second direction D2 by the piston cylinder 116 opposesthe force exerted on the first end 164 of the spool 130 in the firstdirection D1 by the hydraulic circuit 112, the difference in opposingforces determines the maximum hydraulic circuit pressure. Therefore, thegreater the force produced by the piston cylinder 116, specifically onthe plunger 134, in the second direction D2, the greater the maximumpressure in the hydraulic circuit 112. However, the stop structure 132helps to prevent the maximum pressure in the hydraulic circuit 112 frombecoming set at too high of a value. For example, a failure in thecontroller that supplies the current to the solenoid powered valve 114could inadvertently introduce a higher than intended regulated pressureto the second section of the piston chamber 138. Under maximumcompression, the opposing force in in the first chamber 168, which isequivalent to the pressure of the hydraulic circuit 112, must be greaterthan the force exerted by the first spring 118 on the plunger 134 toforce the movement of the spool 130 and de-stroke the pump 102. Forparticular application, too high of a pressure within the hydrauliccircuit 112 could lead to damage of the circuit itself and equipment influid communication therewith. However, to guard against an event likethis, the stop structure 132 prevents the movement of the piston 136,thereby preventing the over compression of the first spring 118 andsetting a maximum pressure for the hydraulic circuit 112. Duringoperation, the force in the second section 172 is transferred to thepiston 136 and then to the spring 118, thereby effecting the compressionof the spring 118. Once the piston 136 has compressed the spring 118 toa point where the piston 136 is positioned within the piston cylinder138 against the positive stop 178 of the stop structure 132, anyadditional force in the direction D2 from the piston 136 is transferredto the stop structure 132 and into the valve block 128, rather than thefirst spring 118.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the claimsattached hereto. Those skilled in the art will readily recognize variousmodifications and changes that may be made without following the exampleembodiments and applications illustrated and described herein, andwithout departing from the true spirit and scope of the followingclaims.

What is claimed is:
 1. A pump control assembly for controlling avariable displacement hydraulic pump, the pump control assemblycomprising: a valve block defining a spool bore having a central boreaxis, the valve block also defining a pump output port, a pumpdisplacement control port, and a tank port; a spool mounted within thespool bore, the spool having a first end and an opposite second end, thespool being movable within the spool bore along the central bore axisbetween a first position where the tank port is in fluid communicationwith the pump displacement control port and a second position in whichthe pump output port is in fluid communication with the pumpdisplacement control port, the spool moving in a first direction alongthe central bore axis when moving from the first position toward thesecond position, and the spool moving in a second direction along thecentral bore axis when moving from the second position toward the firstposition, the first and second directions being opposite from oneanother; a first chamber positioned at the first end of the spool, thefirst chamber being in fluid communication with the pump output port soas to be configured to receive a pump output pressure from the variabledisplacement pump when the pump control assembly is installed on thevariable displacement pump, wherein when the pump output pressure isapplied to the first chamber, the pump output pressure applies a pumpoutput pressure force to the spool in the first direction; a secondchamber positioned at the second end of the spool; a piston positionedwithin the second chamber so as to divide the second chamber into afirst section and a second section, the first section being positionedbetween the piston and the second end of the spool, the piston beingmovable within the second chamber along the central bore axis; a firstspring positioned within the first section of the second chamber fortransferring a piston force in the second direction from the piston tothe spool; a second spring positioned within the second section of thesecond chamber for applying a pre-load force to the piston for biasingthe piston in the second direction; a proportional pressure reducingvalve mounted within the valve block, the proportional pressure reducingvalve being operable in a first state where the tank port is in fluidcommunication with the second section of the second chamber and a secondstate where the pump output port is in fluid communication with thesecond section of the second chamber, the proportional pressure reducingvalve being configured to convert the pump output pressure into areduced pressure that is provided at the second section of the secondchamber, wherein the reduced pressure at the second section of thesecond chamber acts on the piston to apply a reduced pressure force tothe piston in the second direction, and the magnitude of the reducedpressure output from the proportional pressure reducing valve beingdirectly proportional to a current provided to a solenoid of theproportional pressure reducing valve; and a stop structure having apositive stop that stops movement of the piston in the second directionalong the central bore axis at a stop position that defines a maximumthreshold for the piston force transferred by the first spring in thesecond direction from the piston to the spool, the stop position beingadjustable along the central bore axis to adjust the maximum thresholdof the piston force.
 2. The pump control assembly of claim 1, whereinthe stop structure is accessible from outside the valve block to allowthe stop position to be adjusted.
 3. The pump control assembly of claim1, wherein the valve block includes a first side and an opposite secondside, wherein the first chamber is closed by a first plug mounted at thefirst side of the valve block, and the second chamber is closed by asecond plug mounted at the second side of the valve block.
 4. The pumpcontrol assembly of claim 3, wherein the second spring is capturedbetween the piston and the second plug, and wherein an axial position ofthe second plug along the central bore axis can be adjusted to adjustthe pre-load force applied to the piston by the second spring.
 5. Thepump control assembly of claim 2, wherein the stop structure is threadedwithin the valve block and is turned within the valve block to adjustthe stop position.
 6. The pump control assembly of claim 5, wherein alock nut is provided for locking the stop structure at a desired stopposition.
 7. The pump control assembly of claim 3, wherein the stopstructure is mounted within a stop structure bore defined by the valveblock, wherein the stop structure bore extends from the first side ofthe valve block to the second chamber, and wherein the stop structure isaxially movable within the stop structure bore to adjust the stopposition.
 8. The pump control assembly of claim 7, wherein the stopstructure is rotated about a central axis of the stop structure toadjust the stop position.
 9. The pump control assembly of claim 8,wherein stop structure includes an adjustment screw threaded within thestop structure bore, and wherein an end of the adjustment screw formsthe positive stop.
 10. The pump control assembly of claim 9, wherein ahead of the adjustment screw is accessible at the first side of thevalve block.
 11. The pump control assembly of claim 10, wherein the stopstructure bore is parallel to the spool bore.
 12. The pump controlassembly of claim 1, wherein the proportional reducing valve includes areducing valve spool linearly movable between a first positioncorresponding to the first operating state and a second positioncorresponding to the second operating state, wherein the solenoid actson a first end of the reducing valve spool, and wherein the reducedpressure acts on a second end of the reducing valve spool.